Ultrashort laser pulses have been widely used in basic studies for several decades. Recently there has been interest in developing methods of nonlinear coherent microscopy for chemically-selective imaging of biological systems. In recent years, coherent anti-Stokes Raman scattering (CARS) has attracted a lot of attention since it allows one to receive chemically selective vibrational information and has several advantages in comparison with fluorescent and spontaneous Raman scattering based methods.
CARS is a third order nonlinear optical process involving three laser beams: pump, Stokes and probe beams with frequencies ωp, ωs and ωpr, respectively, as reflected in the energy level diagram of FIG. 1. These three beams interact with a sample and generate an anti-Stokes field with frequency ωas=ωp+ωpr−ωs higher than any excitation frequencies. CARS can therefore be detected in the presence of photon-induced fluorescence. Because CARS is a coherent process, the CARS signal is much larger than the spontaneous Raman scattering signal and, in addition, it has spatial (directional) selectivity defined by a phase-matching condition. The phase-matching condition requires the sum of wave vectors of incoming waves (pump and probe) to be equal to sum of wave vectors of outgoing waves (Stokes and anti-Stokes), which means that the laser beams have to be properly aligned.
When the phase-matching condition is satisfied, one has to address a second important aspect of CARS microscopy—namely, designing pulses that provide the maximum coherence on the particular vibrational transition of the target molecule(s) in the sample being analyzed.